Field of the Invention and Related Art Statements
The present invention relates to a method of hot isostatic pressing after a ceramic formed body after the body is set in a glass capsule thereby obtaining a ceramic sintered body of high density. Hereinafter, this method will be referred to as a glass-capsule HIP method.
It is well known that, in the glass-capsule HIP method, all the surfaces of the ceramic formed body to be sintered are surrounded by a glass film having gastight characteristics, and then a hot isostatic pressure such as 100.about.3000 atm at 1000.degree..about.2300.degree. C. is applied to the ceramic formed body covered with the glass film so as to obtain a ceramic sintered body of high density.
In this case, the ceramic powders are formed with 2.about.5 wt % polyvinyl alcohol (PVA) to maintain a forming strength of the ceramic formed body. Therefore, in order to remove the polyvinyl alcohol, the ceramic formed body is kept in an atmosphere at 500.degree. C., 10.about.20 hrs. Moreover, when the ceramic formed body is fired, 5.about.20 wt % of MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 are added as sintering agents, and a matrix ceramic formed body such as silicon nitride powders always includes 2.about.3 wt % of SiO.sub.2.
However, if the ceramic formed body, in which several materials mentioned above remain, is used for the glass-capsule HIP method, the following drawbacks occur and thus a ceramic sintered body of high density can not be obtained.
(1) Since PVA is added, about 1 wt % of carbon remains in the ceramic formed body. The residual carbon and the residual oxide then reacted at high temperatures, and consequently CO gas or CO.sub.2 gas is generated in the glass-capsule. Therefore, the glass-capsule is expanded from the inside due to the generation of CO or CO.sub.2 gas, and, in an extreme case, the glass-capsule is broken.
(2) The sintering agents such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 easily absorb a water component, and the water component once absorbed in the sintering agents can hardly be removed. If once removed, the sintering agents easily absorb a water component in the air again during storage in a storehouse. Therefore, the residual water component in the ceramic formed body is vaporized at high temperatures, and thus the glass-capsule is expanded from the inside. In an extreme case, the glass-capsule is also broken.
The above problems are unique to the glass-capsule method, and thus, in the normal HIP operation using no glass-capsule, the thus generated H.sub.2 O, CO, CO.sub.2 gases are mixed with the atmosphere and do not affect the sintering operation. Therefore, in order to solve the problems mentioned above, the known methods for making high density ceramic sintered bodies are not applied to the glass-capsule HIP method.